They did it because they were being thorough.
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Daniel R HicksSep 7 '12 at 9:36

1

@StephenTouset I should explained further that my seat post is coated in something black and also the frame is painted. If they were both bare metal it would be understandable
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Mark WSep 7 '12 at 13:51

2

The inside of the seat tube is almost certainly not painted, and any substance your seatpost is likely to be coated in to make it black will almost certainly not prevent seizure.
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Stephen TousetSep 7 '12 at 21:47

3

The black coating on an aluminum seatpost is probably black anodizing, which is only atoms thick.
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Daniel R HicksSep 8 '12 at 0:40

5 Answers
5

On old steel bikes a good reason would be to prevent the seatpost from rusting onto the frame, thus preventing saddle height adjustment.

More recently I understand from my LBS that you do it to prevent water from seeping into the frame along the (imperfectly sealed) seatpost. Depending on your frame you could end up carrying one or more kilo's of water with you if you have a lot of rainy rides.

In addition to the seizure problem that others have mentioned, greasing the seatpost also prevents another problem: creaks.

I once spent weeks chasing down a creak that I initially thought was coming from the crank or bottom bracket. After disassembling and reassembling all that stuff, then tightening almost every single bolt on the bike, someone suggested that the creak could be coming from the seatpost rubbing inside of the seat tube and I should grease it. Bingo! Problem solved.

That bit of grease on the seat post into the seat tube will help "seal" the seat tube from getting any moisture into the bike frame. It may also keep the seat post from seizing in the seat tube. It does happen if the seat isn't moved for years.